In an aircraft, the cabin air conditioning system serves to supply the cabin and the cockpit with fresh air and recirculated air. In addition, the cabin air conditioning system must ensure that also during flight at high altitude a pressure predominates in the cabin and the cockpit which corresponds substantially to the air pressure on the ground. The cabin air conditioning system uses as an air source bleed air from the engines or from an auxiliary engine (APU: auxiliary power unit) which for example can be arranged at the rear end of an aircraft. The temperature of the bleed air is usually approximately 150 to approximately 200° C. It is cooled in a heat exchanger (the so called main heat exchanger or primary heat exchanger). The heat exchanger is supplied with external air which during flight is available as ram air in a ram air channel. On the ground a fan generates an air flow through the heat exchanger to cool the bleed air.
The ram air channel normally comprises a so called NACA ram air inlet channel (NACA: National Advisory Committee for Aeronautics), a diffuser, optionally a ram air channel plenum and a ram air outlet channel. Between the diffuser and the ram air outlet channel is arranged the heat exchanger of the cabin air conditioning system described initially. Furthermore, in the ram air channel a fan can be arranged which on the ground ensures an air flow through the heat exchanger of the cabin air conditioning system.
During flight external air enters the ram air channel via the ram air inlet channel. The dynamic part of the pressure in the diffuser is partly converted into a static pressure which slows down the flow. Consequently, in relation to ambient pressure, a static positive pressure is produced called the ram pressure. The ram pressure causes a flow of external air through the heat exchanger of the cabin air conditioning system. The flow of external air through the ram air channel is controlled by a ram air inlet channel flap. The ram air outlet channel is also usually fitted with a flap. The relatively widely opened flap of the ram air outlet channel generates a reduced pressure in relation to ambient pressure as external air flows around it. Thus, the positive pressure present on the inlet side at the heat exchanger and the reduced pressure present on the outlet side lead to a differential pressure which determines an air mass flow through the heat exchanger. The flaps on the ram air inlet channel and the ram air outlet channel are controlled by actuators.
When the aircraft is on the ground, there is no flow of external air. A fan, for example the fan of the ACM (air cycle machine), causes a flow in the ram air channel so that external air is made available to the heat exchanger of the cabin air conditioning system to cool the hot bleed air.
An aircraft can comprise at least one further ram air channel with which at least one further heat-loaded component of the cabin air conditioning system can be cooled and/or the installation space of the heat-loaded component can be ventilated. The function of this ram air channel substantially corresponds to that of the ram air channel previously described, with the exception that there is no heat exchanger and no ram air outlet channel provided in the ram air channel, but the ram air channel opens into a distribution line which supplies cooling air to a heat-loaded component of the cabin air conditioning system and/or passes air into an installation space in which the heat-loaded component of the cabin air conditioning system is arranged.
Because of leakages, fuel and/or fuel vapours can collect or form in the installation space. The fuel and/or fuel vapour can enter the installation space from the tank. Therefore the installation space of the heat-loaded component must be ventilated, i.e. vented. The ventilation of the installation space should prevent that a combustible mixture can form in the installation space. If the cabin air conditioning system with the heat-loaded component is to be switched on, first the installation space of the heat-loaded component must be ventilated since, also with the air conditioning system switched off, fuel and/or fuel vapour can collect in the installation space. Cooling of the heat-loaded component can also protect the structure against over-heating.
The air flow generated by a fan in the ram air channel must be monitored. If the fan fails, the cabin air conditioning system must be switched off, since a combustible mixture can form in the installation space of the heat-loaded component. This monitoring can be carried out by an air conditioning system switch-off device on which a software is running. The air conditioning switch-off device for example analyses, whether the fan speed lies within a desired range. This air conditioning system switch-off device is an additional device constituting an additional fault source which should be avoided.
This system for ventilating the installation space of the heat-loaded component and for cooling the heat-loaded component is also called UBV (unpressurised bay ventilation). The aim is to guarantee that despite the potential presence of fuel vapours, for example from leakages, no combustible mixture is formed in the installation space of the cabin air conditioning system. The surfaces of some devices of the cabin air conditioning system have hot surfaces, whereby combustion could be provoked. Furthermore, the ventilation system serves to cool the air in the installation space of the cabin air conditioning system.
In an aircraft important devices are always designed redundant. Thus, in an aircraft there are provided two ram air channels to cool the heat exchanger of the cabin air conditioning system and two ram air channels with a distribution line to cool at least one heat-loaded component and to ventilate the installation space of the heat-loaded component. Furthermore, four fans are present to generate an air flow on the ground. The channels and the fans require space, must be maintained and increase the mass of the aircraft.
The object of the invention is to reduce the space requirement of a cooling system.